If you did not file your tax return, the IRS has created a substitute tax return in their computer system using income information submitted to them by your employers, banks, brokers, and so on. This substitute tax return most likely does not include any deductions or credits that you might be eligible for, because the IRS is not required to award any deductions or credits unless you apply for them in writing. As a result, the substitute tax return probably shows you as owing more taxes than you would owe if you filed your return.
As noted by the other answers, this substitute tax return will also accrue interest and penalties.
Then, if you file a 2018 tax return with a refund claim, it is almost guaranteed that the IRS will withhold the refund automatically to offset the taxes, interest, and penalties that have accrued to the substitute 2017 tax return.
And of course, the IRS can bill you at any time for the taxes, interest, and penalties due from 2017. Once they bill you, you will have 30 days in which to respond in writing with a tax return to show the amount you actually owe. If you do not respond, then the substitute return becomes your official return and you lose your right to appeal or correct the assessment of taxes, interest, and penalties.
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